Load specific dictionary used by SnapshotMeta

Just getting this ready for when we merge `repo-commits`.
I think there are 2 situations. One where you're about to "update" the dictionary, so if there is none yet, you don't want an error, this is normal. So that's `ResultTIO[Option[Dictionary]]`. And one situation where you expect the dictionary to already exist and you just want it, that's `ResultTIO[Dictionary]`. There is a pattern like that in different places where we have `lastxxxOrFail` methods.